1. Field of the Invention
The present invention relates to a process for producing a cordierite honeycomb structural body used as a catalyst carrier for an exhaust gas purification catalyst in an automobile engine such as an internal combustion engine, and to a molding aid used for molding of the honeycomb structural body.
2. Description of the Related Art
With the toughening of exhaust gas standards for automobile engines in recent years, there has been a demand for more rapid activation of exhaust gas purification catalysts in order to reduce hydrocarbon emissions immediately after engines are started. One means of rapid activation of catalysts that has been considered is to lower the heat capacity by reducing the thickness of the cell walls in cordierite honeycomb structural bodies acting as catalyst carriers, but with narrowing of the cell walls, an inconvenience has resulted as the cell walls break during extrusion molding of the honeycomb structural body. This occurs because coarse grains in the starting material of the honeycomb structural body clog the lattice-like slits or introduction port of the extrusion mold for molding, thus inhibiting provision of the starting mixture, and this requires prior removal of the coarse grains in the starting material.
As concerns the particle size of the starting material powder, Japanese Unexamined Patent Publication No. 8-112528 teaches that lattice defects of molded bodies can be reduced by limiting the ratio (maximum particle size of starting material powder)/(slit width of extrusion mold for molding) to 1/3. However, because honeycomb structural bodies formed under these conditions have low void volume, the effect of reduced heat capacity is less significant, while the catalyst carrier property is also weaker. In addition, because of a larger thermal expansion coefficient, there is also a problem of lower thermal shock resistance.
Narrowing the cell walls of a honeycomb structural body also tends to result in molding defects known as cell wrinkles in the honeycomb structural body. These will be explained below. When a honeycomb structural body is extrusion molded, the starting mixture is first molded into a round bar form, and the round bar is extrusion molded into a honeycomb form. A screw-type tug mill such as shown in FIG. 1A is usually used for the round bar molding, in order to obtain a homogeneous round bar of the starting mixture for molding. A screw-type tug mill has upper and lower level screws 1, 2, and is provided with a vacuum chamber 3 and a strike-through roller 4 between the upper level screw 1 and the lower level screw 2. A resistance plate 5 is fitted in front of the lower level screw to create a uniform flow of the starting mixture and, as shown in FIG. 1B, the resistance plate 5 has a structure with a plurality of round holes 5 opened in a disk. The starting mixture which is kneaded by the upper and lower screws 1, 2 and passes through the resistance plate 5 is thus converted into a plurality of bar-shaped bodies which are introduced into a round bar mold 6 and are bonded together into a round bar as the cylinder size of the mold 6 narrows toward the tip.
The starting mixture used for molding of the round bar has conventionally been a cordierite-converted starting material of talc, kaolin, etc. with a water-soluble polyhydric alcohol added as a molding aid, but the cohesion is insufficient between the starting mixture after it has passed through the resistance plate 5 of the screw-type tug mill, and a starting mixture interface corresponding to the shape of the resistance plate 5 is formed on the round bar. This starting mixture interface presents almost no problem when molding a ceramic honeycomb structural body with a cell wall thickness of 100 .mu.m or greater, and causes no visible molding defects. When the cell wall thickness is less than 100 .mu.m, however, it has been found that cell wrinkles are generated at the sections corresponding to the starting mixture interface, wherein the cells of the ceramic honeycomb structural body ripple in the direction of extrusion. It is thought that this is caused because the thin cell wall results in a higher molding pressure, leading to precipitation of moisture, etc. at the starting mixture interface and greater flowability of the starting mixture near the interface; thus the difference in the flowability at the other sections where the flowability of the starting mixture does not change produces a change in the cell formation rate of the ceramic honeycomb structural body, thus leading to generation of the cell wrinkles.
Japanese Unexamined Patent Publication No. 7-138076 discloses a method of adding emulsified wax and methyl cellulose, as molding aids for reduced frictional resistance between the starting mixture and the mold wall surface, to improve molding defects such as stripping of the outer perimeter surface or cell wrinkles in ceramic honeycomb structural bodies. With this method, however, it has not been possible to eliminate the starting mixture interfaces on round bars, and thus a difference in flowability between the starting mixture interface and the other sections is produced. Consequently, while some effect of fewer cell wrinkles is seen by lowering the frictional resistance between the starting mixture and the mold wall surface, it is not possible to completely eliminate cell wrinkles.
The prior art processes, therefore, have been associated with the problem of molding defects such as cell breakage and cell wrinkles when the cell wall thicknesses of cordierite honeycomb structural bodies are reduced. It has also been necessary to position the catalyst carrier as close as possible to the engine in order to take advantage of the engine exhaust gas temperature for rapid activation of the catalyst. However, positioning the catalyst carrier close to the engine leads to the problem of cracking due to thermal shock as the catalyst carrier is exposed to sudden temperature variations. Improving the thermal shock resistance of the catalyst carrier requires a reduction in its thermal expansion coefficient, and specifically, to prevent cracking near the engine it is necessary for the cordierite honeycomb structural body to have a thermal expansion coefficient of 1.0.times.10.sup.-6 /.degree. C. or smaller.
It is therefore an object of the present invention to obtain a honeycomb structural body with a low cell wall thickness, with good moldability, wherein cell breakage is prevented without reducing the void volume and cell wrinkles caused by the starting mixture interface formed on the round bar are prevented, and to obtain a honeycomb structural body with excellent thermal shock resistance having a thermal expansion coefficient of 1.0.times.10.sup.-6 /.degree. C. or smaller.
The first aspect of the invention, designed to solve the problem of cell breakage when the cell wall thickness has been reduced, is a process for producing a honeycomb structural body composed mainly of cordierite which comprises adding a molding aid to a powder of a cordierite starting material, kneading the mixture and extrusion molding it with an extrusion molding die with honeycomb-shaped slits, and then firing it, the process being characterized in that the maximum particle size of the powder of the cordierite starting material is limited to no greater than 85% of the slit width of the extrusion molding die, at least talc is used as the cordierite starting material, and the mean particle size thereof is 5 m or greater.
If the maximum particle size of the cordierite starting material is smaller than the slit width of the extrusion molding die the starting material particles should not clog between the slits or at the slit introduction port; however, clogging in fact occurs if it is only slightly smaller than the slit width. The present inventors have found cell breakage due to clogging of the starting material particles can be eliminated if the particle size of the starting material is restricted so that the maximum particle size is no greater than 85% of the slit width. However, if the particle size of the starting material is simply reduced, the void volume is also smaller and an effect of lower heat capacity by thickness reduction cannot be achieved. For a lower heat capacity it is preferred for the void volume to be greater than 30%, and according to the first aspect talc with a mean particle size of at least 5 .mu.m is used for this purpose. Talc forms voids by being melted during firing, thus providing an effect of increased void volume. Thus, while talc with a small particle size will disappear by contraction during the firing, if the mean particle size of the talc is at least 5 .mu.m the disappearance of voids can be prevented, to give a honeycomb structural body with a void volume of greater than 30%.
In addition, the thermal expansion coefficient of the cordierite honeycomb structural body can be controlled by utilizing the orientation of the plate-crystal talc particles lined up along the cell walls of the honeycomb during molding of the honeycomb, and it has been found that a larger mean particle size results in easier orientation and a smaller thermal expansion coefficient. Specifically, if the mean particle size of the talc is at least 5 .mu.m, it is possible to limit the thermal expansion coefficient of the cordierite honeycomb structural body to no greater than 1.0.times.10.sup.-6 /.degree. C., to thus provide increased thermal shock resistance.
Thus, according to the process of the first aspect, it is possible to avoid cell breakage without reducing the void volume, while it is also possible to lower the thermal expansion coefficient to 1.0.times.10.sup.-6 /.degree. C. or smaller. It thus becomes possible to obtain an easily moldable honeycomb structural body with thin cell walls, a good catalyst carrying property, a low heat capacity and excellent thermal shock resistance.
A lubricant/humectant is preferably added as a molding aid at 2-5 wt % to 100 wt % of the cordierite starting material. Addition of a lubricant/humectant further improves the effect of preventing cell breakage. This is because insertion of a substances with low frictional resistance between the starting material particles increases the distance between the starting material particles, having the effect of lowering the frictional resistance and preventing clogging of the starting material particles in the slits, and therefore the lubricant/humectant is preferably added at a total of 2 wt % or greater. However, if the amount of the lubricant/humectant added is too great the hardness of the starting mixture will be lowered, thus rendering it difficult to maintain the shape of the molded honeycomb structural body. Thus, a range of 2-5 wt % is best to achieve both lower frictional resistance and shape retention.
It is preferred to add a binder as the molding aid at 3-9 wt % to 100 wt % of the cordierite starting material. Addition of a binder will also provide an effect of reducing the frictional resistance to prevent clogging of the starting material particles and thus prevent cell breakage, similar to addition of the lubricant/humectant. The binder may be added in an amount in the range of 3-9 wt % to achieve this effect with shape retention.
The molding aids used to overcome the problem of cell wrinkles when the cell wall thickness is reduced are preferably a mixture of a water-soluble polyhydric alcohol derivative and a polyhydric alcohol, and they are preferably added so that the mixing ratio is represented by the following equation:
mixing ratio=polyhydric alcohol/(water-soluble polyhydric alcohol derivative+polyhydric alcohol) is in the range of 0.895-0.995.
Addition of the mixture of the water-soluble polyhydric alcohol derivative and the polyhydric alcohol with this mixing ratio to the cordierite starting material will improve the cohesion of the starting mixture when shaping the round bar for molding of the honeycomb structural body. It will thus become possible to eliminate the starting material interface in the round bar to prevent generation of cell wrinkles caused thereby. The mixing ratio may be at least 0.895 in order to achieve this effect, but if the mixing ratio exceeds 0.995 the hardness of the starting material will be lowered thus reducing the shape retention property which holds the shape, and leading to generation of warps and the like; the mixing ratio should therefore be in the range of 0.895-0.995.
Thus, according to the method of using a water-soluble polyhydric alcohol derivative and a polyhydric alcohol in this proportion, it is possible to eliminate the starting material interface in the round bar and to mold a honeycomb structural body with a narrow cell wall thickness without producing cell wrinkles.
The second aspect of the invention is a molding aid added to the starting material for a honeycomb structural body during molding of the honeycomb structural body, characterized by comprising a mixture of a water-soluble polyhydric alcohol derivative and a polyhydric alcohol such that the mixing ratio represented by the following equation:
mixing ratio=polyhydric alcohol/(water-soluble polyhydric alcohol derivative+polyhydric alcohol) is in the range of 0.895-0.995.
By using a molding aid containing the mixture of the water-soluble polyhydric alcohol derivative and polyhydric alcohol for molding of a honeycomb structural body, such as a cordierite honeycomb structural body, it is possible to improve the cohesion of the starting material for molding of the honeycomb structural body and prevent generation of cell wrinkles caused by the interface of the starting mixture on the round bar. If the mixing ratio of the water-soluble polyhydric alcohol derivative and polyhydric alcohol is within the range specified above, it is possible to eliminate the interface of the starting mixture on the round bar and thus prevent generation of cell wrinkles in the honeycomb structural body in cases where the honeycomb structural body has narrow cell walls.